The present invention relates to both the qualitative and quantitative composition of catalysts for the low-temperature pyrolysis of hydrocarbon-containing polymer materials, said catalysts being mainly intended for use in the recycling of waste rubber materials.
Hereinafter:
The term of xe2x80x9chydrocarbon-containing polymer materialsxe2x80x9d, as used herein, refers preferably to composition materials capable of swelling in organic solvents, which contain the binders produced by either polymerization or co-polymerization of alkenes, alkadienes, alkatriens, cycloalkenes, cycloalkadienes and cycloalkatriens and/or derivatives thereof, both alone and with a use of cross-linking agents, and usually not-readily-removable extenders, but preferably to compounded rubber based on vulcanized natural and/or synthetic rubber.
The term of xe2x80x9cwaste rubber materialsxe2x80x9d, as used herein, refers to worn mechanical rubber articles irrespective of their design, initial sizes and designation.
It is evident for those skilled in the art that waste rubber materials are the most routine form of hydrocarbon-containing polymer materials. Therefore, all that which will be said hereinafter about waste rubber materials might be attributed to other hydrocarbon-containing polymer materials capable of swelling in organic solvents. It as also evident for specialists that used tires of vehicles and, especially, metal-cord tires are presently the principal source of waste rubber materials. Therefore, all that which will be said hereinafter about waste rubber materials of used tires might be attributed to other waste rubber.
It is generally known that waste rubber materials can serve as a source of valuable recycled resources. However, their recovery is impeded by high strength of chemical bonds in macromolecules of vulcanized rubber and by difficulties of rubber separation from reinforcing steel usually present in mechanical rubber articles. In addition, most of the known methods and means for rubber destruction are dangerous because of the environmental contamination with sulfur compounds, carcinogenic carbon black and certain other toxic substances.
It is also generally known that used tires and, especially, metal-cord tires are one of the most serious contaminant of the environment even in the non-recycled form. Thus, 250 million tires are only worn annually in US during the recent 10 years, and their total number dumped has exceeded 3 milliards. Therefore, the problem of worn tires utilization is the more acute, the more economically developed is a country and the less is its territory.
As applied to tires with textile cord, whose mechanical crushing and grinding is acceptable both energetically and ecologically, the indicated problem has been and is being solved by means of:
either the mechanical separation of crushed tires into rubber crumbs and cellulose-containing fiber materials, and their separate utilization (see, for example, U.S. Pat. No. 1,607,291),
or the rubber depolymerization, for example in a steam pressurized autoclave (U.S. Pat. No. 2,447,732) or using an apparatus for pyrolysis under vacuum (U.S. Pat. No. 4,740,270), etc.
As applied to worn metal-cord tires, purely mechanical crushing of said tires and grinding of their fragments at the ambient temperature is disadvantageous energetically.
Not long ago, in a number of countries, the problem of such tires disposal has been partly solved by a way extremely dangerous ecologically, i.e. by combustion (see, for example, the paper xe2x80x9cTire Recycling Plant Tire Upxe2x80x9d in Modern Tire Dealer, 1987, No.8, p. 6). At that, metal cord was turn into rust of indefinite composition, which was practically nonrecoverable because of substantial cost of the collection, delivery and preparation to introduce them into the metallurgical process.
The shortcomings indicated have been excluded in the processes that provide for staged mechanical grinding of worn tires with at least single freezing.
Thus, the processes are known that include the trimming of tire beads, preliminary crushing of the trimmed tires into fragments, usually of about 3 cm in thickness, and:
either the milling of fragments at the ambient temperature, magnetic separation of metal, sieve fractionation of rubber particles with the isolation of (1) commercial fine crumbs and (2) grains with linear sizes of preferably 2 to 7 (and no more than 15) mm, which are frozen in liquid nitrogen and then further milled to obtain rubber crumbs with linear sizes of 0,2 to 2,0 mm (xe2x80x9cGummibereitungxe2x80x9d, 1987, Bd.63, No.10, S.102-104);
or the freezing of rubber fragments prior to their milling (xe2x80x9cGummibereitungxe2x80x9d, 1987, Bd.63, No.10, S.97-100; U.S. Pat. No. 5,385,307) with the subsequent separation of metal and fractionation of rubber particles by size.
At that, ecological safety and high quality of the target products of, tire disposal are reached at the expense of low production rate and high specific power inputs. Therefore, the practical use of such processes is limited.
The thermal-electric separation of metal cord from rubber, according to the application DE 2900655, requires for preliminary trimming of tire beads and is only efficacious when cord wires pierce rubber all the way through, from bead to bead. Otherwise, neither warming of entire metal-cord mass up to the temperature of thermal rubber destruction, nor burning-out of all its layers adjacent to cord wires is possible. In addition, the method indicated results in discharge into the air of toxic rubber destruction products.
The induction warming of metal cord (see, for example, the application DE 3911082; the application EP 0 478 774) eliminates the need for trimming beads; however, it increases an output of toxic rubber destruction products.
The electropulse destruction of cord wire (RU Patent 2050287) is based on discharging either a capacitor battery or an accumulation reactive LC-circuit on the metal cord. This process is characterized by a short-term heat emission with a high density of heat rating along boundaries of crystal grains of the metal and, consequently, by the bursting nature of cord wire destruction. At that, a considerable part of fine metal particles fly out of rubber tearing it to fragments of different sizes.
Being ecologically safe, this process does not ensure the efficacious separation of metal and rubber irrespective of the power applied to metal cord, and its practical use is hampered by the following circumstances:
firstly, because of unevenly trimmed beads and/or the above indicated position of a considerable part of cord wires within rubber thickness, effective contact not always can be ensured between a capacitor battery and metal cord with the resulting incomplete destruction of cord wires, not to mention the release of all metal particles out of rubber thickness;
secondly, the electromagnetic (transforming) transfer of power from a reactive LC-circuit to cord wires is the less effective, the more is mass (and inductivity) of the metal cord enclosed in a disintegrating tire.
The efficacious, from the point of purity of the products obtained, and ecologically safe separation of rubber and metal cord during the destruction of integral worn tires of any dimension-types is possible in principle with the use of a powerful laser pulse delivery into the liquid surrounding a mechanically distended tire (see publication WO 97/44171 of International Application PCT/UA96/00011). In that case, rubber becomes fragile at the room temperature and separated completely from metal cord, which can be easily utilized metallurgically.
However, the coefficient of efficiency of such a process is hardly in excess of 1% because of low (usually no more than 5%) efficiency of lasers, which precludes its use in practice.
Therefore, such methods and means are preferable for recycling tires, especially those with metal cord, that do not require any preliminary separation of rubber and cord wires and allow a set of commercial valuable secondary products to be obtained out of waste rubber, and utilize cord wires practically with no additional processing.
Most of such processes are based on the low-temperature (no more than 500xc2x0 C., usually within a range of 200 to 400xc2x0 C.) catalytic pyrolysis of waste rubber and only require cutting tires, including big tires with metal cord, into several large pieces.
Parameters, in particular temperature and pressure, and results of the catalytic pyrolysis of waste rubber depend greatly on the catalysts used. A critical index of their quality is activity that can be evaluated by their specific consumption per mass unit of the waste pyrolyzed, by minimal allowable temperature and pressure of the pyrolysis, by a degree of the polymer organic component conversion into low-molecular products, by a qualitative composition of such products, by their proportions in the mixture, and by their suitability for any subsequent use (with their minimal possible, or with no, additional processing prior to sale).
For example, U.S. Pat. No. 3,996,022 discloses a catalyst for the pyrolysis in a form of a fusion of halides, for example zinc chloride and stannum chloride. At a temperature of above 300xc2x0 C. and a positive pressure, it ensures the transformation of waste rubber into a mixture of gaseous hydrocarbons, low-sulfur motor fuel and solid carbon residue.
However, the specific consumption of such catalyst is rather high, the process of its usage runs under sufficiently strict conditions, and the solid residue, which is produced in a great amount, is only suitable as a substitute for domestic coal, and can only be removed with difficulty out of the pyrolysis apparatus.
Similar results are obtained with the use of a powdery catalyst based on an iron compound (usually an oxide) with an auxiliary metal compounds of the Periodic Table, Group IV (Cr, Mo, W, Se, Te) in the process of pyrolysis of a mixture of hydrocarbon fuel and crushed waste rubber in the presence of hydrogen at a temperature of 350 to 500xc2x0 C. and a pressure of 10 to 30 MPa (U.S. Pat. No. 4,251,500; JP Patent 52-125088).
At that, the achievement of a certain reduction in the solid residue amount and an increase in the motor fuel produced is associated with the need for specific safety measures against fire and explosion.
A catalyst-for the waste rubber pyrolysis, disclosed in U.S. Pat. No. 5,286,374, is made of mica such as muscovite, sericite and biotite. It allows the process of pyrolysis of worn metal-cord tires to be led at a reaction temperature of 230 to 400xc2x0 C. under a pressure of 1.0-2.5 atmospheres.
Such a catalyst allows gaseous hydrocarbons, a wide range of liquid fuels, carbon black and a small amount of solid carbon residue, predominantly on cord wires, suitable for utilization as a metallurgical raw material to be produced in the absence of hydrogen, that is under significantly safer conditions.
However, its consumption rate amounts 2 to 3% of the initial rubber mass, whereas a total output of solid products of the pyrolysis reaches 43%. At that, carbon black comprising only a part of these products is markedly contaminated with an admixture of the catalyst, which hampers its utilization.
A catalyst disclosed in UA Patent No. 10442 is the closest prior art for the proposed catalyst. It is produced as a product of the gaseous carbon oxide decomposition over iron scale and, in essence, represents a homogenous iron-carbon composition material in the form of ultra-dispersed iron particles and microscopic carbon particles. This catalyst, hereinafter referred to as xe2x80x9ciron-carbon componentxe2x80x9d, is added into hydrocarbon solvents of the used machine oil type in an amount of no less than 0.2%, and preferably from 0.5 to 1.0% of solvent""s mass. Waste rubber materials are left in this mixture till they swelled no less than 1.1 times, an excess of the solvent with the catalyst admixed is decanted for repeated use, while the waste rubber materials swelled are exposed to the pyrolysis at a temperature of 150 to 450xc2x0 C., preferably near 200xc2x0 C., in nitrogen current.
Consequently, the specific consumption rate of this catalyst amounts to 0,002-0.005% of the rubber mass. As a result, free-of-admixtures solid products of the pyrolysis in the form of carbon black with specific surface of 80 to 100 m2/g are suitable for the production of new general mechanical rubber goods, at least in the mixture of 50/50 with fresh carbon black.
However, the entire condensate of liquid products of the pyrolysis produced with a use of the catalyst described constitutes, in essence, black oil fuel having a boiling onset temperature of over 150xc2x0 C., a boiling end temperature of near 400xc2x0 C. and a flash temperature of over 100xc2x0 C., which contains over 90% of paraffin-like hydrocarbons with the chain length of above C5. Besides, in the case of using the catalyst described, the bound sulfur contents in the pyrolysis products not infrequently are below 1.0%. These undesirable results can be explained by still insufficiently high activity of the powdery catalyst in reactions of destructing the macromolecules containing hydrogen and carbon.